The present invention relates to a method for obtaining well defined edge radii on cutting tool inserts by electropolishing technique.
Inserts for chip forming machining made of cemented carbides or titanium-based carbonitrides (cermets) have at least one main cutting edge and a connecting nose (corner). Such inserts are produced by the powder metallurgical methods of milling of powders of the hard constituents and binder phase, pressing to form bodies of a desired shape and finally sintering the pressed bodies. The pressing is generally done by tool pressing between two opposing punches in a die. As a result of the pressing operation, the inserts have rather sharp edges. In addition, because of the small gap, a few microns wide that always exists between the punches and the die wall, the insert edges also have burrs. Such edges break too easily when used.
Therefore, after sintering, the inserts are subjected to an edge rounding operation including mechanical methods such as lapping, tumbling, brushing or blasting. These operations, however, are difficult to control with desirable accuracy. For this reason, the edge rounding values usually range between 30 and 75 .mu.m on cemented carbide inserts for a majority of machining applications. Smaller edge rounding values are generally not possible to obtain with mechanical methods. Also, the edges often get defects in the initial stage of the mechanical operation. These detects disappear during the continued treatment provided that the final edge rounding obtained is larger than the defect size.
A finer edge rounding, however, means lower cutting forces. The choice of edge rounding is a compromise between the desired edge strength and acceptable cutting forces. For certain cutting operations such as threading and machining of heat resistant materials, aluminum or cast iron, low cutting forces are desirable. However, the above mentioned methods for edge rounding are generally not useful, at least on a large, industrial scale.
Electrolyte smoothing or deburring is a commonly employed technique. Two well-known processes are called electrochemical deburring and electropolishing. U.S. Pat. No. 4,405,422 discloses methods for electrolyte deburring of copper or copper alloys and U.S. Pat. No. 4,411,751 of steel or aluminum alloys. However, when subjecting materials with phases of differing chemical properties such as cemented carbide to chemical treatments. The metallic binder phase is often dissolved first, resulting in a porous surface layer with reduced strength and often containing portions comprising several grains that have disappeared, (so-called pitting). It is therefore essential that an electrolyte is used which provides an even removal of material, essentially without depth effect. An example of this is U.S. Pat. No. 5,380,408, (our reference: 024000-819)incorporated by reference herein, which discloses a method for removing cobalt from the surface of cemented carbide using an electrolyte of sulphuric and phosphoric acids. This method, however, but does not generate edge rounding since it only removes cobalt, leaving the carbide or carbonitride grains intact.